This invention relates to flat panel display devices and, more particularly, to the creation of an adequate number of spacers between the anode and cathode thereof to maintain substantially uniform spacing therebetween.
Flat field emission cathode displays typically have a cathode electron emitting surface (also referred to as a base electrode, baseplate, emitter surface, or cathode surface) and a corresponding anode display screen (also referred to as an anode, cathodoluminescent screen, display face, faceplate, or display electrode) with an evacuated cavity therebetween. There is a relatively high voltage differential (e.g., generally above 300 volts) between the cathode emitting surface and the display screen. A full description of a field emission display can be found in U.S. Pat. No. 4,940,916, the disclosure of which is incorporated herein by reference. It is important to prevent catastrophic electrical breakdown between the electron emitting surface and the anode screen by maintaining substantially uniform spacing therebetween while avoiding any structure which might contribute to arcing. At the same time, the narrow spacing between the plates and the thin structure of the plates are necessary to maintain the desired overall thinness of the FED display. The spacing also has to be substantially uniform for constant image resolution and brightness, as well as to avoid display distortion, etc.
Small area flat displays (e.g., those which are approximately 1" diagonal) generally do not require spacers, since glass having a thickness of approximately 0.0401" can support the normal atmospheric pressure load without significant bowing. However, as the display area increases, spacer supports become more important. For example, a flat display having a 10" diagonal measurement will have sufficient force exerted on it by atmospheric pressure to cause significant bowing of 0.040" thick glass. Thus spacers play an important role in maintaining the structural integrity and substantial uniform parallel spacing across large area, light weight, flat panel displays.
Spacers are incorporated between the faceplate and the cathode emitter plate. The spacers enable the thin, lightweight substrates to withstand normal atmospheric pressure thereby allowing the display area to be increased with little or no increase in either substrate thickness or overall thickness of the display.
Spacers must conform to certain parameters. The spacers must: 1) be sufficiently non-conductive to prevent catastrophic electrical breakdown between the cathode array and the anode, in spite of the relatively close inter-electrode spacing (which may be on the order of 200 .mu.m), and relatively high inter-electrode voltage differential (which may be on the order of 300 or more volts); 2) exhibit mechanical strength such that they prevent the flat panel display from collapsing under atmospheric pressure; 3) exhibit stability under electron bombardment, since electrons will be generated at each of the pixels; 4) be capable of withstanding "bakeout" temperatures of around 400.degree. C. that are encountered in creating the high vacuum between the faceplate and backplate of the display; and 5) be small enough in cross section so as to not to interfere with display orientation.
There are several drawbacks to the spacers currently employed in FEDs and the methods of applying them. Methods employing screen printing, stencil printing, or the like suffer from the inability to provide a spacer having a sufficiently high aspect ratio. The spacers formed by these methods can easily be either too short for the high voltages (allowing arcing) or too wide (interfering with the display image). Forming spacers by reactive ion etching and plasma etching of deposited materials suffer from slow throughput (i.e., time of fabrication), slow etch rates, and etch mask degradation. Lithographically defined photoactive organic compounds result in the formation of spacers which are not compatible with the high vacuum conditions or elevated temperature characteristics in the manufacture of field emission displays. The formation of spacers is described in U.S. Pat. Nos. 4,923,421; 5,205,770; and 5,232,549, the disclosures of which are incorporated herein by reference.